Specially considering that they are pronounced similar if not the same.
I know "nigga" is also considered offensive by a lot of people but I think on general terms, you're likely to offend more saying "nigger" rather than "nigga".
Why is this?
The thing that needs to be understood is that, from an "official etymology" standpoint, "nigga" did not come from the offensive term "nigger", but rather is (it was claimed) simply a more "casual" and "countrified" version of "negro" (which was at one time considered entirely non-offensive).
This was a topic of some discussion and controversy back in the 60s, IIRC, as prominent white Southern politicians would regularly use a word similar to "nigra" when speaking to the public and the press. And, if challenged, they would claim that they were saying "negro", just in their dialect. (Inside the mayor's chambers, of course, they said "nigga", if not actually "nigger".) While white politicians learned other terms, the term "nigra"/"nigga" was adopted by black people, at least partially in mocking imitation of white politicians.
Of course, to what extant any of this is actually true is lost in the cultural history of the South, too jumbled by conflicting prejudices to ever sort out.